Online bookselling giant, AbeBooks did a great video on the cause of the distinctive smell of old books. Here's an excerpt of the description that they posted to accompany their video on YouTube.
A physical book is full of organic material that reacts with heat, light, moisture and - mostly importantly - the chemicals used in its production. The smell comes from the reaction of the organic material to these factors.
Chemists at University College, London have investigated the old book odor and concluded that old books release hundreds of volatile organic compounds into the air from the paper. The lead scientist described the smell as "A combination of grassy notes with a tang of acids and a hint of vanilla over an underlying mustiness."
There you have it. Inhale deeply the scent of old books...
Have a fantastic Thursday!